A spark plug for use in a combustion apparatus, such as an internal combustion engine, includes, for example, a center electrode extending in the axial direction, an insulator provided externally of the outer circumference of the center electrode, a cylindrical metallic shell mounted to the outside of the insulator, and a ground electrode extending from a forward end portion of the metallic shell and bent toward the center electrode. Also, in order to improve ignition performance and erosion resistance, there is proposed a technique for joining a noble metal tip formed of a noble metal alloy to a forward end portion of the center electrode.
Furthermore, in recent years, elongating the noble metal tip along the axial direction has been proposed (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2009-158343). Such proposition has been made for, for example, the following reason.
In a spark plug of such a type that the forward (or distal) end surface of a ground electrode faces the side surface of a distal end portion of a noble metal tip to thereby generate spark discharges across a spark discharge gap between the two members substantially along a direction orthogonal to the axis (a so-called lateral discharge type), if the ground electrode and a fusion zone of joining a center electrode and the noble metal tip are close to each other, an abnormal spark discharge may be generated between the fusion zone and the ground electrode, potentially resulting in a deterioration in durability. In this regard, by means of the noble metal tip being elongated, a sufficient distance along the axial direction can be secured between the fusion zone and the ground electrode, whereby there can be more reliably prevented the generation of an abnormal spark discharge and, in turn, a deterioration in durability.
Also, in a spark plug of such a type that a distal end portion of a ground electrode faces the distal end surface of a noble metal tip to thereby generate spark discharges across a spark discharge gap between the two members substantially along the axial direction (a so-called parallel electrode type), by means of the noble metal tip being elongated, the position of ignition can be projected closer to the center of a combustion chamber, whereby ignition performance can be improved. That is, in view of improvement of durability and ignition performance, various types of spark plugs could employ elongation of the noble metal tip along the axial direction.